A photocoupler essentially consists of a light emitting element and a light receiving element which are electrically isolated but optically coupled. When an electric signal is entered into the photocoupler, the electric signal is first converted into an optical signal by the light emitting element. This optical signal is reconverted into an electric signal by the light receiving element. Then, the reconverted electric signal is released by the photocoupler.
The photocoupler is used for example when an external input/output device is connected to a programmable controller. The photocoupler makes it possible to connect the programmable controller and the external input/output device even if the respective input and output levels are different. This becomes possible because the two can be kept electrically isolated from each other when connected via the photocoupler.
Normally, over 64 photocouplers are required in order to connect a programmable controller and an external input/output device. To reduce the number of parts, an optoelectronic device comprising a plurality of photocouplers sealed in a package is currently in use.
An example of an optoelectronic device is shown in FIGS. 10 and 11. This optoelectronic device comprises four photocouplers in a package 3 made of resin having light interrupting properties. Each photocoupler has a light emitting element 1 and a light receiving element 2, disposed so as to face each other, the pair of the light emitting element 1 and the light receiving element 2 being sealed in an inner package 4a made of translucid resin.
As shown in FIG. 12, the light emitting element 1 requires a positive terminal 1a and a negative terminal 1b, and the light receiving element 2 requires a ground terminal 2a, an output terminal 2b, and a power terminal 2c. Consequently, the optoelectronic device has a total of 20 terminals.
In order to miniaturize the optoelectronic device, it is desirable that the number of terminals be as small as possible. Accordingly, it is conceivable that, instead of providing the ground terminal 2a and the power terminal 2c on each of the light receiving elements 2, a ground terminal and a power terminal common to all the light receiving elements 2 be provided thereon.
The number of the terminals used can be reduced, if for example, as shown in FIG. 13, ground electrode sections of the four light receiving elements 2 are respectively connected to a lead 5 provided in the package 3 and the lead 5 is drawn out as a ground terminal common to all the light receiving elements 2 (a ground electrode section is provided on each of the four light receiving elements 2). Miniaturization thereby becomes possible.
However, if the four photocouplers are disposed in the inner package 4b made of the translucid resin, light (shown for example by the arrow in FIG. 13) from one of the light emitting elements 1 is incident not only upon the facing light receiving element 2 but also upon the neighboring light receiving elements 2. As a result, there is a danger of the neighboring light receiving elements 2 malfunctioning. In other words, there exists a problem of crosstalk between photocouplers.
In order to prevent this from occurring, it is conceivable, as shown in FIGS. 14 and 15, that a light path 4c made of translucid resin be provided only between each of the pairs of the facing light emitting elements 1 and the light receiving elements 2, the entire body being sealed in the package 3 made from the resin having light interrupting properties.
However, during manufacturing, the light paths 4c may become joined, as shown by alternate long and short dash lines in FIG. 14. That is, there is still a possibility of light leaking from the light emitting elements 1 to neighboring light receiving elements 2.
In order to prevent this from happening, a light interrupting wall made of resin having light interrupting properties can be provided between neighboring light receiving elements 2, the translucid resin being introduced thereafter. However, since this increases the number of processing steps, a new problem arises in that the manufacturing cost increases.
A lead frame is used in the manufacture of the optoelectronic device. For simplicity, FIG. 16 shows a lead frame having two light receiving elements 2 provided thereon.
The lead frame is made from a thin metal plate and comprises a tie bar 9 and four leads 5, 6, 7 and 8 which extend from the tie bar 9. The light receiving elements 2 are fixed onto the lead 5 by die bonding. Wiring is carried out by wire bonding. That is, a ground electrode of each of the light receiving elements 2 is connected to the lead 5 by a wire 11a. A power electrode of each of the light receiving elements 2 is connected to the lead 8 by a wire 11b. Output electrodes of the light receiving elements 2 are connected respectively the leads 6 and 7 by a wire 11c, one output electrode being provided on each light receiving element 2. When the tie bar 9 is cut away from the lead frame, a ground terminal 5a common to the two light receiving elements 2, two output terminals 6a and 7a, and a power terminal 8a common to the two light receiving elements 2 are achieved.
Bent sections 10 (see FIG. 11 or FIG. 15) are provided in the leads 5, 6, 7, and 8 in order to provide a space between each facing pair of the light emitting element 1 and the light receiving element 2. However, if an error occurs in the bend angle of any of the bent sections 10, a predetermined space can no longer be maintained. This results in a fall in the output of the light receiving element 2 or a fall in the withstand voltage of the optoelectronic device. Moreover, the longer the lead 5, the greater is the displacement of the light receiving element 2. Therefore, it is difficult to make the lead 5 longer than shown, if the optoelectronic device is to operate normally. This means that it is difficult to have three or more light receiving elements 2 on the lead 5. In other words, it is difficult to manufacture an optoelectronic device provided with three or more photocouplers.
Further, although an optoelectronic device having two or more lead frames is conceivable, setting two or more lead frames accurately and fixing these in translucid resin is problematic. Manufacturing efficiency is lowered and a high-precision optoelectronic device cannot be achieved.